1.Leaf area index (LAI), a measure of canopy density, is a key variable for modelling and under-standing primary productivity, and also water use and energy exchange in forest ecosystems. How-ever, LAI varies considerably with phenology and disturbance patterns, so alternative approaches toquantifying stand-level processes should be considered. The carbon isotope composition of soilorganic matter (d13CSOM) provides a time-integrated, productivity-weighted measure of physiologicaland stand-level processes, reflecting biomass deposition from seasonal to decadal time scales.2.Our primary aim was to explore how well LAI correlates withd13CSOMacross biomes.3.Using a global data set spanning large environmental gradients in tropical, temperate and borealforest and woodland, we assess the strength of the correlation between LAI andd13CSOM; we alsoassess climatic variables derived from the WorldClim database.4.We found that LAI was strongly correlated withd13CSOM, but was also correlated with MeanTemperature of the Wettest Quarter, Mean Precipitation of Warmest Quarter and Annual Solar Radi-ation across and within biomes.5.Synthesis.Our results demonstrate thatd13CSOMvalues can provide spatially explicit estimates ofleaf area index (LAI) and could therefore serve as a surrogate for productivity and water use. Whiled13CSOMhas traditionally been used to reconstruct the relative abundance of C3versus C4species,the results of this study demonstrate that within stable C3-orC4-dominated biomes,d13CSOMcanprovide additional insights. The fact that LAI is strongly correlated tod13CSOMmay allow for amore nuanced interpretation of ecosystem properties of palaeoecosystems based on palaeosol13Cvalues.